In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warnings:

drivers/watchdog/machzwd.c: In function ‘zf_set_timer’:
./arch/x86/include/asm/io.h:355:14: warning: this statement may fall through [-Wimplicit-fallthrough=]
 #define outw outw
drivers/watchdog/machzwd.c:80:53: note: in expansion of macro ‘outw’
 #define zf_writew(port, data)  { outb(port, INDEX); outw(data, DATA_W); }
                                                     ^~~~
drivers/watchdog/machzwd.c:179:3: note: in expansion of macro ‘zf_writew’
   zf_writew(COUNTER_1, new);
   ^~~~~~~~~
drivers/watchdog/machzwd.c:180:2: note: here
  case WD2:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>